Air-sealing a metal chimney
Hello GBA community, I have a question about air-sealing an ICC Excel chimney servicing a woodstove in Manitoba, Canada:
I am installing the chimney using a square cathedral support which doubles as an attic radiation shield. I have already purchased the support and chimney components, and am ready to install the chimney. It is going into a cathedral ceiling insulated with dense-pack cellulose. The roof will be vented with the vent channel outboard of the roof sheathing. The cathedral support passes through both air barriers: ice & water shield on top of the ceiling boards at the interior, and ZIP-taped plywood sheathing at the exterior.
I phoned ICC this morning to get some guidance on what sealants or tapes to put in contact with the cathedral support, and was told that despite having zero clearance to combustibles required (i.e. wood framing and non-foam insulation in contact with the cathedral support is acceptable), no tape or sealant of any kind can be used. The support person with whom I spoke acknowledged the existence of fire-rated caulks but was adamant that there was no possible way for me to maintain the continuity of my air barrier using the conventional cathedral support that I have.
She advised me that ICC makes an insulated support with a flange meant to seal to an interior membrane, which can only be installed to a level surface such as a flat ceiling or built-out box, and encouraged me to refer to the website, which says, “The Insulated Round Support (ERDSI) is designed for installations in energy efficient houses which have tightly sealed vapour barriers. The RDSI is fully insulated and has a flange to permit an air tight seal to the vapour barrier.” She also told me that if it was possible to air-seal the other supports, the product description would have explicitly stated so.
Is that true? I can’t believe that the typical support used in the vast majority of chimney installations would be completely incompatible with any attempt at air-sealing, and I feel like the support person I talked to was out to lunch, based on other descriptions at GBA and elsewhere. If she’s right, what are my options? I live in the sticks, and any ICC product is a special-order item that I can’t return, so I would need to eat the square support and buy this second insulated support in its place. Alternatively, should I be building a plywood chase around the support, and then air-sealing that so that the air leaking around the cathedral support leaks straight up to the vent channel and at least doesn’t infiltrate the insulated cavity? Lastly, if she is wrong, and I can apply some sealants or tapes to the cathedral support, what can I use that will also make a durable seal to the granular surface of the ice-and-water shield?
As usual, I’ve taken a lot of words for a small problem. Thanks in advance for your time!